Honestly, I think Toxic Thread just wins for a multitude of reasons.
First, it's on an obscure and bad Pokemon. Ariados isn't a Pokemon you first think of even at the below depths of a metagame, I bet.
Secondly, the move is just... awful. Like let's break it down. We have a move that regular poisons, so not even Toxic poison. We also have a move that reduces Speed by 1 stage only. Meanwhile, String Shot reduces Speed by 2 stages. So we have a move that is inferior to a few separate moves. It isn't even like Spider Web where it's just another Mean Look clone. This is a move that somehow takes components from a stat and status effect of similar moves that exist and somehow manages to be fucking worse. You can't even meme Venoshock and Sniper. Ariados has 60 base SpA.
By the way, all of this ignores that Ariados learns Agility. Yes Ariados is piss slow to use Agility, but before we consider Toxic Thread, much less String Shot, we need to consider this first.
Like yeah moves like Rototiller are very forgettable and are garbage in their own right. Yet somehow this move finds a way to be downright awful in a very oddly spectacular way. The worst part? This isn't even a move like Constrict, which is one of the worst moves in the game. In fairness many Pokemon learn Constrict very early (when the move existed), so while garbage you could argue it was just one of those semi-useless earlygame moves. Spinirak and Ariados don't even learn these moves until Level 54 and 63, respectively.